This invention relates to a method and apparatus for controlling the flow of a fluid through a conduit and the positioning of an element in a conduit. More particularly, the invention relates to a method and apparatus for controlling the flow of blood through a conduit and the positioning of a catheter in a catheter sheath. More particularly, this invention pertains to an elastic valve apparatus for preventing leakage of blood through an introducing catheter sheath inserted in a vessel and around a catheter inserted in the catheter sheath.